1. Field of the Invention
The present invention relates to plastic lenses and processes for preparing the same. More specifically, one aspect of the present invention relates to a method for preparing plastic lenses having a high-refractive index, high surface accuracy and excellent optical properties. Another aspect of the present invention relates to a process for preparing a sulfur-containing urethane resin lens which comprises cast polymerizing a mixture of a polyisocyanate having two or more isocyanato groups, a polythiol having two or more thiol groups and a specific surface active agent, and to a lens prepared by the process.
2. Description of the Related Art
Plastic lenses that are used in optical applications must be completely free from bubbles and must have high surface accuracy and optical homogeneity such that no strain can be found by an optical strain meter, and must also have high transparency. Conventionally, diethylene glycol bis(allylcarbonate) (hereinafter abbreviated as DAC), PMMA, and polycarbonates have been used as materials for lenses. Generally, in the process for preparing plastic lenses, cutting and abrading, hot press molding, cast molding, injection molding or the like have been employed. When a thermosetting resin such as DAC is used, cast molding is employed.
In this case, the releasability between the lens and the mold is important. If the release is premature, the lens surface is disordered and the lens tends to exhibit strain. Conversely if the release is difficult, exfoliation of the mold takes place, and exfoliated pieces of the mold stick to the surface of the lens. Although it is known that butyl phosphate is used as an internal release agent for DAC, generally it is not preferable to use an internal release agent in view of the physical properties of the lens (see "Polymer Digest" by Seiichi Mima, 3, 39 (1984).
On the other hand, as a sulfur-containing urethane resin for a lens and a lens prepared therefrom, U.S. Pat. No. 4,689,387 has suggested a thiocarbamic acid S-alkyl ester resin and a lens which can be obtained from a polyisocyanate and a polythiol.
When the thiocarbamic acid S-alkyl ester lens is molded, it is usually impossible to release the polymerized lens from the mold due to excessive adhesion between the lens and the mold.
Therefore, as a method of improving the releasability thereof, a process in which an external release agent is used was proposed by U.S. Pat. No. 4,689,387, and a process in which a polyolefin resin mold is used was proposed by Japanese Patent Laid-Open Publication No. 236818/1987.
However, a process wherein a fluorine type external release agent or a silicone type external release agent is used to form a release film on the mold is accompanied by such problems that it is difficult for the thickness of the release film to be uniform, which makes it difficult to maintain the surface accuracy of the lens. Further, in such a process, a part or all of the release film migrates to the surface of the polymerized lens or into the inside of the polymerized lens and deteriorates the surface state of the lens. In particular, the surface becomes nonuniform and the lens becomes turbid.
In addition, the adhered release agent on the surface of the lens must be removed therefrom, and it is necessary to treat the mold with the release agent every time the mold is used. Thus, the process is intricate and cannot provide high productivity, which means that it is very uneconomical.
As a consequence, a process using an external release agent which involves high cost cannot be employed as an industrial production process.
On the other hand, in a process wherein a mold of a polyolefin resin is employed, the resin mold deforms at elevated temperatures which causes large profile irregularities on the molded lens and thus is impractical for use in the field of optical lenses wherein high surface accuracy is required.